Cryptococcus neoformans is an important fungal pathogen that is a relatively frequent cause of life-threatening infections in patients with impaired immunity. Current therapy is unsatisfactory because the available antifungal agents often fail to eradicate the infection in immunocompromised individuals with cryptococcosis. Although cellular immunity is critically important for controlling and eradicating C. neoformans infections there is overwhelming evidence from several independent laboratories that humoral immunity can also make a decisive contribution to host defense. A novel approach to therapy is to administer antibodies to the polysaccharide capsule as adjunctive immunotherapy. A murine monoclonal antibody (mAb) that binds to capsular polysaccharide is currently in clinical evaluation. The relationship between antibody structure and efficacy against C. neoformans appears to be extremely complex. Antibody specificity, isotype and amount have each been shown to be important variables in protective efficacy. In the past funding period this research program identified several amino acid residues that conferred a protective specificity, discovered a new mechanism for IgM-mediated phagocytosis, and re-discovered prozone-like effects in a passive protection experiments. This competing renewal application proposes to continue molecular studies to ascertain the structural determinants and mechanisms of antibody efficacy against C. neoformans. The proposed work takes advantage of a well defined system to explore important questions of antibody function. Three specific Aims are proposed: 1. To identify the VH structural motifs required for protection against Cryptococcus neoformans; 2. To establish the mechanism and consequences of complement-independent IgM-mediated phagocytosis of C. neoformans; and 3. To establish the mechanism for prozone-like effects in passive antibody protection experiments. These studies are expected to yield new insights on fundamental aspects of antibody function as well as practical information for the development of second generation antibody reagents for human therapy.